The present invention relates to a multi-color image forming apparatus, which may be used in an electrophotographic apparatus, such as a copy machine, a printer, a facsimile machine and so on.
In a multi-color image forming apparatus, a charging unit, an exposing unit, a first developing unit, a second unit developing unit, a pre-transfer unit and a transfer unit are arranged around a drum on which a photoconductor is formed. In order to obtain a tri-level, two-color image, the photoconductor is initially charged by the charging unit, and then a charged latent image of the tri-level electric potential is formed on the photoconductor by the exposing unit, as shown in FIG. 6.
In FIG. 6, a portion L.sub.1 is a first latent image portion at the highest electric potential level V.sub.0 (representing the level to which the surface is charged by the charging unit) and a portion L.sub.2 is a second latent image portion at the lowest electric potential V.sub.r, and to these first and second portions a first color toner and a second color toner, to be described later, are caused to adhere, respectively. An intermediate electric potential V.sub.W is a background level. The second latent image portion L.sub.2 is formed by a negative image exposure of strong intensity, and the latent image portion at the intermediate electric potential level V.sub.W is formed by a positive image exposure of weak intensity.
The first latent image portion is developed by the first developing unit, and the second latent image portion is developed by the second developing unit. That is, the first color toner having a polarity opposite to the charge polarity of the photoconductor is caused to adhere onto the first latent image portion L.sub.1 by setting the developing bias voltage of the first developing unit to V.sub.b1 (normal development), and the second color toner having the same polarity as the charge polarity of the photoconductor is caused to adhere onto the second latent image portion L2 by setting the developing bias voltage to the second developing unit to V.sub.b2 (reversal development).
The first color toner and the second color toner, applied in the manner described above, are unified to either a positive or a negative polarity by the pre-transfer charger and are transferred onto a print medium by the transfer unit. The transferred toner is then heat-melted and fixed on the print medium.
The technology relating to such a two-color image forming process, as described above, is proposed in Japanese Patent Application Laid-Open No. 48-57637 and in U.S. Pat. No. 4,078,929, and the technology relating to the pre-transfer charging for charging the positive and the negative charged toner to either a positive or negative polarity is disclosed in Japanese Patent Application Laid-Open No. 48-53729.
In a multi-color image forming apparatus, an organic photoconductor (OPC), which can be negatively charged, is used for the photoconductor. However, the initial electric potential charge is likely to be higher in a tri-level method than in a bi-level method in which a charged latent image of two electric potential levels is formed. When the initial electric potential charge becomes high, the amount of ozone generation is likely to be increased. Further, in a single-color image forming mode, in which a charged latent image of bi-level electric potential is formed and a single-color image is obtained, it is necessary to perform pre-transfer charging depending on the polarity of the toner to be used, and, accordingly, the amount of ozone generation is likely to be increased even more than when pre-transfer charging is not performed.